Spontaneous Selective Deposition of Iron Oxide Nanoparticles on Graphite

Iron oxide nanomaterials participate in redox processes that yield ideal properties for earth abundant catalysts. Fabricating nanocatalysts for such applications requires detailed knowledge of the deposition and growth. We report a simple method for spontaneous iron oxide nanoparticle growth on HOPG defects from a metal precursor solution. To study the nucleation and growth of iron oxide nanoparticles, tailored defects were created on the surface of HOPG using various ion sources that serve as the target sites for iron oxide nucleation. After solution deposition and annealing, the iron oxide nanoparticles were found to nucleate and coalesce by 400 °C. Nanoparticles on the sp3 carbon sites enabled the thermal migration and aggregation to larger particles, resulting in two domains of particles on the graphite surface. The iron oxide nanoparticles were found to have a Fe3+ oxidation state with two different oxygen species, Fe-O and Fe-OH by XPS. STEM imaging and EDS mapping confirmed that the majority of the nanoparticles grown were converted to hematite after annealing to 400 °C. A mechanism of spontaneous and selective deposition on the HOPG surface and transformation of the iron oxide nanoparticles is proposed for the growth and transformation to hematite. These results suggest a simple method for growing nanoparticles as model catalysts.